4-capillary viscometer module

About the 4-capillary viscometer

What a differential viscometer measures

Intrinsic viscosity, written [η], is a measure of the volume per unit mass that a polymer occupies in solution. More simply, it is a measure of molecular density.

A differential viscometer gets at this by comparing the viscosity of the sample with the viscosity of the solvent. The 4-capillary design does that with two differential pressure measurements.

Getting intrinsic viscosity from the signals

The viscometer does not give [η] on its own. To calculate it, the DP and IP signals from the viscometer are used together with the signal from the RI detector, which supplies the concentration.

The viscometer response is proportional to three things: the intrinsic viscosity, the concentration, and the injection volume. A detector constant ties them together. That constant is found during calibration, along with the constants for the other detectors and the delay volume between detectors. With those known, [η] can be calculated from the viscometer response and the concentration at each slice of the chromatogram.

Molecular weight and intrinsic viscosity

Intrinsic viscosity and molecular weight are related through the Mark-Houwink equation:

log [η] = log K + a log M

This has the same form as a straight line, y = mx + b, so a plot of log intrinsic viscosity against log molecular weight gives a line whose slope is a and whose intercept is log K. The slope, a, is related to the structure of the polymer, and K is related to its density. When intrinsic viscosity and molecular weight are both measured, the Mark-Houwink a and K parameters can be reported, and branching is one of the results that can then be reported.

Universal calibration

Conventional calibration uses a series of narrow molecular weight standards and relates molecular weight to retention volume. Universal calibration instead plots the log of [η] times Mw against retention volume, and the 4-capillary viscometer is what provides the [η].

The advantage is that polymers of different architecture and chemistry can fall on the same curve, so the calibration is not tied to one polymer type. It is also the most time-consuming of the calibration types, because it needs detector response factors and a column calibration before it can be used.

Where the viscometer fits among the detectors

The viscometer adds a property to a system. It does not replace the concentration detector. Each type of calibration uses a different set of detectors:

• Conventional: RI or UV
• Absolute: RI and light scattering
• Universal: RI or UV, and a viscometer
• Triple: RI, light scattering and a viscometer
• Quad: RI, UV, light scattering and a viscometer

With a viscometer in the system, the results that can be reported include intrinsic viscosity, both as a bulk value and as a distribution, the Mark-Houwink a and K parameters, and branching.